1. Field of the Invention
The present invention relates to a fixing apparatus for fixing a transferred image onto a sheet of paper. More particularly, the present invention relates to a heating roller of a fixing apparatus, which is capable of blocking power being supplied to the fixing apparatus when a temperature of the fixing apparatus increases to more than a predetermined temperature, and a method for manufacturing an electrode for use with the same.
2. Description of the Related Art
Generally, a fixing apparatus fixes an image transferred from a photosensitive medium onto a sheet of paper with heat generated by a heating roller and pressure applied by a pressing roller. The heating roller typically includes a halogen lamp, a heating coil, and a resistance heating element or the like which acts as a heat source.
FIG. 1 is a view showing an example of a fixing apparatus having a heating roller 80 which generates heat by using a heating coil. Referring to FIG. 1, the heating roller 80 includes an aluminum pipe 10, insulators 20, heating coils 30, an OFC (Oxygen Free Copper) pipe 50, electrodes 40, brushes 45 and a thermostat 60.
The aluminum pipe 10 forms the outer surface of the heating roller 80, and is manufactured from aluminum that has high heat conductivity. Both ends of the aluminum pipe 10 are supported by bearings 15. Moreover, the surface of the aluminum pipe 10 is typically a Teflon coated layer 12 in order to have non-stick properties with respect to a toner.
The insulators 20 insulate the heating coils 30 in the aluminum pipes 10 from the aluminum pipe 10 and the OFC pipe 50, and comprise external layer insulators 22 to insulate the heating coils 30 and the aluminum pipe 10 and internal layer insulators 24 to insulate the heating coils 30 and the OFC pipe 50. The external layer insulators 22 and internal layer insulators 24 are generally made of mica or some other suitable material.
The OFC pipe 50 is made of OFC having high heat conductivity, and contains a working fluid 52. Both ends of the OFC pipe 50 are closed.
The electrodes 40 are installed at both ends of the aluminum pipe 10, and connected to the heating coils 30. The brushes 45 are connected to an outer circumference of the electrodes 40 so that AC (alternating current) power can be supplied to the electrodes 40 even when the heating roller 80 is rotated. The brushes 45 are further connected to an AC control 70, and the thermostat 60 is connected to an electric wire connecting the brushes 45 and the AC control 70. The thermostat 60 operates to block power being supplied to the fixing apparatus when the surface temperature of the aluminum pipe 10 rises to more than a predetermined temperature, and is installed at an appropriate place for measuring the surface temperature of the aluminum pipe 10. In this example, the thermostat 60 is secured with a bolt (not shown) to an injection molded polymer frame that wraps the fixing apparatus.
In addition, a pressing roller 90 that rotates and presses against the aluminum pipe 10 is installed below the heating roller 80 as shown.
The process of heating the heating roller 80 having the above structure will now be described.
When the electrical power is supplied from the AC control 70, AC power is supplied to the brushes 45. After the power is supplied to the brushes 45, the power is transmitted to the electrodes 40 installed at both ends of the aluminum pipe 10, and the power transmitted to the electrodes 40 is provided to the heating coils 30 so that heat is generated by the heating coils 30.
Some of the heat generated by the heating coils 30 is transmitted to the aluminum pipe 10 and heats its surface, and rest of the heat is transmitted to the OFC pipe 50. The heat transmitted to the OFC pipe 50 rapidly gasifies the working fluid 52 in the OFC pipe 50 due to the high heat conductivity of OFC. After the working fluid 52 in the OFC pipe 50 is completely turned to saturated vapor, the heat generated at the heating coil 30 is all or substantially all transmitted to the surface of the aluminum pipe 10 and used to settle a toner and increase the temperature of a sheet of paper.
As described so far, when the OFC pipe 50 is used, an image can be instantly fixed as temperature increases by 10° C. per second. In addition, the OFC pipe 50 can rapidly raise the efficiency in which the temperature is increased due to the saturated vapor of the working fluid 52 contained inside, and also prevents the temperature of the aluminum pipe 10 from being rapidly cooled after printing. Thus, power need only be supplied to the heating coil 30 at a frequency of two or three times per sheet of paper. Therefore, power consumption for printing is less than that consumed by an apparatus whose heating roller is heated by a halogen lamp.
When the AC power is not normally controlled with respect to the heating coil 30 due to, for example, malfunction of the AC control 70, the heating coil 30 is constantly heated, and the surface temperature of the aluminum pipe 10 increases. In this event, a bimetal switch of the thermostat 60 that is installed separate from the surface of the aluminum pipe 10 operates to block the AC power. Then, the heating roller 80 can cool when power is not supplied to the heating coil 30 of the heating roller 80.
In other words, if the AC control 70 that controls the supply of AC power malfunctions due to, for example, a malfunction of the CPU of the fixing apparatus. Also, if a photo triac that controls the on/off switching of the AC power supplied to the heating coil 30 of the heating roller in accordance with a control signal from the CPU malfunctions and thus causes an “On” signal to be continuously transmitted to the heating coil 30 even when the CPU outputs an “Off” control signal, the thermostat 60 should sense the rise in the surface temperature of the aluminum pipe 10 and block the AC power being provided to the heating coil 30.
However, the injection molded polymer frame having the thermostat 60 tends to become bent or misformed. Therefore, it is sometimes difficult to maintain the appropriate distance between the thermostat 60 and the heating roller 80. When the distance between the thermostat 60 and the heating roller 80 is not consistent, the surface temperature of the heating roller 80 may not be accurately detected by the thermostat. In other words, due to these variations from unit to unit, the temperature at which the thermostat 60 blocks the AC power being supplied to the heating coil 30 is not consistent for each unit produced. When the thermostat 60 operates too late because the distance between the thermostat 60 and the heating roller 80 is too large, the heating roller 80, the pressing roller 90, and frame of the fixing apparatus might become melted or give off fumes. Thus, the possibility of fire can increase, and a user might experience some anxiety.
In an attempt to avoid the above problems, a thermostat that contacts the surface of the heating roller 80 can be used. However, this type of sensor can cause abrasion and breakage of the Teflon coated layer 12 on the surface of the aluminum pipe 10 of the heating roller 80.